This invention has applicability to the general field of technology wherein it is desired to have, in a single apparatus, a variable volume air pump and a peristalic pump, both pumps being driven by the same prime mover such as an electric motor.
This invention, more specifically, is directed to the field of "soft" ice cream machines wherein air pumps are used to supply air which is combined with liquid ice cream mix to yield an air/liquid mixture. When a constant speed electric drive motor is energized, then the liquid ice cream mix is typically supplied at a constant rate by a suitable pump such as a peristalic pump. The air and liquid ice cream mix are combined in a mixer means such as a "T" connection and the resultant air/liquid mixture is pumped to a freezer-storage tank unit where the mixture is stored under pressure and the temperature of the mixture is reduced to a pre-selected point; the mixture then may be selectively dispensed into soft ice cream cones or other containers for customer consumption. It is very important to have accurate control of the ratio of air to liquid in the mixture for both the control of the quality, i.e., the "taste" of the soft ice cream and to provide a compressible air-liquid ice cream mixture so that the mixture can be stored under pressure in the freezer-storage tank unit. The prior variable volume air pumps used in soft ice cream machines have not been fully satisfactory. Additional information is helpful in understanding the shortcomings of the prior art pumps. It is important to have the air-liquid ice cream mixture under pressure in the freezer tank unit (as aforesaid) so that the pumps only be intermittently operated, i.e., continuous operation of the pumps is not desirable. A typical prior art system has a tank pressure sensor switching means which initiates pump action at 17 psi.+-.3 psi (above atmospheric pressure) and which stops the pumping at 23 psi.+-.3 psi. Also, at initial start up, the freezer tank is empty and at atmospheric pressure. For example, a prior art variable volume pump piston-type pump is characterized by having a significant variable clearance between the top of the piston head (when the piston head is at the maximum travel of the "up-stroke") and the cylinder head; thus there will be air remaining in the cylinder after the piston has completed its "up-stroke"; this creates a major problem because of the above described "system" characteristics. To explain, upon initial start-up of both the air pump and peristalic pump, since there is no tank "back pressure" for the pump to work against, the pump will deliver a very large volume of air greatly in excess of the desired amount for the desired ratio of air to liquid. Each successive compression stroke contributes (on a diminishing basis) to the incorrect ratio of air to liquid. For this type of air pump there is only one tank pressure at which the desired air/liquid mix ratio is achieved. Thus there is a problem upon initial start up and the problem continues after the tank is full as the system cycles between the low pressure start and high pressure stop as described above. Further, when the aforesaid prior art pump is subjected to a variation in atmospheric pressure, the air flow rate will vary because of the expansion or contraction of the air gap between the top of the piston and the cylinder head. For example, the pumps might be manufactured at a factory at an elevation 1,000 ft. above sea level, but the pumps might be used at a customer location at an elevation 5,000 ft. above sea level. Both of these problems are because of the air in the gap described above. Prior art air pumps with "zero gaps" are known; however, such pumps are not variable volume when driven at constant speed. The present invention provides, in part, a variable volume, "zero gap" air pump which is especially well suited for "soft" ice cream machine applications in combination with a peristalic pump.